I don't think writing an history based manga is any easier than a complete fictional one. Hara is bound by RL events/deaths/wars that took place, so readers like you who show interest in Chinese history can look up and call the deaths of certain characters or winners of a certain war. The author is brilliant in making it as tense as he can despite the outcome being obvious.
A fictional story has creational freedom. Oda can be as unpredictable as he wants to be because we can't predict shit unlike in Kingdom.
Politics in Kingdom is also handled very good in Kingdom
Kingdom is breath taking ... and I'm not saying a historical story can't be as good as a fantasy one
and Coalition arc was nothing less than a master piece
but a fantasy story can push all limits and achieve more ... and one piece plot is truly high tier even if we compare it to top novels
the world building + on going plots and story arcs are just massive and unreal
sure we have weak arcs like wano (I'm not a fan of wano) but balance of small drama and big drama in one piece and art of story telling is just madness
I mean look at this chapter ... Oda is using information from chapter 1 to build up people in chapter 1018. this is mad tier story telling
one piece plot is a masterful piece of art that can tie everything together while Kingdom plot is just about THAT arc (mostly) and is
much more focused
that's why Kingdom arcs are more enjoyable you can say
while one piece is bigger and more complex
since I love Kingdom I don't want to downrate it but I don't think it's even fair to compare it to one piece
one piece best is yet to come btw